Many industrial, medical, biological, and research situations require measurements of concentrations of selected gases in flowing gas streams. Such measurements may comprise determining the absolute concentration of a selected gas in a flowing gas stream, or the difference in concentration of a selected gas between two flowing gas streams. The latter is typically employed in research and biological applications where, for example, quantification of the O.sub.2 consumption or production by biological material is required. Measurements involve placing biological material in a cuvette containing a known concentration of O.sub.2, and monitoring changes in the atmosphere within the cuvette with time. Preferably, an open gas exchange system is employed, which involves placing the biological material in a cuvette through which gas of known composition flows at a measured rate. The O.sub.2 concentration of the effluent gas from the cuvette is monitored by an O.sub.2 analyzer, and the difference in O.sub.2 concentration between the input and effluent gases multiplied by the flow rate through the cuvette gives a measure of the rate of O.sub.2 exchange. If the O.sub.2 analyzer used in the open system is itself a flow-through instrument, the O.sub.2 concentration in the effluent gas stream can be monitored continuously, and real-time measurements of O.sub.2 exchange can be performed.
The most accurate method of measuring O.sub.2 exchange in an open flow gas exchange system is to use a differential O.sub.2 analyzer. Such instruments continuously monitor the difference in O.sub.2 concentration between a reference gas stream and a branch of the reference gas stream which has passed through a cuvette containing the biological material under study. Where large oxygen differentials between the reference and sample gas streams occur, sensitivity of the differential analyzer is not critical and instruments such as those containing either paramagnetic O.sub.2 sensors (e.g. the Oxygor.TM. 6N, Maihak AG, Hamburg, Germany) or zirconium oxide sensors (e.g. Model S-3A/II, Servomex Company, MA 02062, USA) may be used. The sensitivities of these instruments are limited; the Oxygor 6N can resolve a minimum O.sub.2 differential of only 100 ppm O.sub.2 when air is used as the reference gas, and under the same conditions the Servomex S-3A/II has an accuracy limit of only .+-.30 ppm O.sub.2 in differential mode (note that 1 PaO.sub.2 is approximately equivalent to 10 ppm O.sub.2). However, neither instrument has the sensitivity required to measure very small O.sub.2 differentials (e.g., less than 10 ppm) that occur when the biological material under study has a low metabolic rate, or the sample is very small. Also, both types of differential O.sub.2 analyzer are essentially laboratory-based intents which are not readily adaptable for field use, as each requires AC power and stable environmental conditions for most accurate function. They also require calibration by laboratory-based calibration systems involving compressed gases and/or gas mixing instruments. This adds to the considerable expense of the analyzers.
The differential O.sub.2 analyzer described in our U.S. Pat. No. 5,542,284, issued Aug. 6, 1996, was developed to overcome the sensitivity limitations of the above-mentioned analyzers. Using O.sub.2 sensors, which are O.sub.2 cells that operate on the principle of a lead-oxygen battery, the prior device permits measurement of as little as 2 ppm differential in O.sub.2 concentration between two flowing gas streams that contain 21% O.sub.2 (210,000 ppm). While the prior analyzer provides for measurement of very small differentials in O.sub.2 concentration, it has other limitations. For example, it is not possible to obtain an absolute measurement of the O.sub.2 concentration in either of the sample and reference gas steams. Since many applications require information about the absolute O.sub.2 concentration in the gas streams, this is a drawback of the prior design.